Water or various gases are used in many types of unclear reactors to remove heat from the reactor core, which is then directly or indirectly used to generate electricity. In a pressurized water reactor (PWR) water circulates between the reactor core and a steam generator in a primary loop. In the steam generator the heat is transferred to a secondary loop of water which forms steam which then runs turbine electric generators. In a boiling water reactor (BWR) the water in the primary loop is under less pressure so that, after heating in the nuclear core, it is in a gaseous form. In other types of nuclear reactors, such as high temperature gas reactors (HTGR), a gas such as carbon dioxide or helium transfers heat from the reactor core to the steam generator.
Regardless of whether the heat transfer medium is water or a gas, however, it picks up contaminants and corrosion products from the metals with which it is in contact. The contaminants are radioactivated in the nuclear core, and then deposit on metal surfaces in the cooling system. These contaminants include chromium which enters the coolant when base metals such as stainless steel or Inconel corrode. Chromium (+6) is soluble (e.g., as dichromate, Cr.sub.2 O.sub.7.sup.--) but chromium (+3) forms an oxide with a spinel structure, which is very difficult to remove from the metal surfaces. Such spinel-like oxides include chromium substituted nickel ferrites, such as Cr.sub.0.2 Ni.sub.0.6 Fe.sub.2.2 O.sub.4, which tend to form under the reducing conditions found in pressurized water reactors. The deposits can also contain nickel ferrite, hematite, magnetite, and various radionuclides. Hematite, Fe.sub.3 O.sub.4, and, to a lesser extent, nickel ferrite, NiFe.sub.2 O.sub.4, tend to form under the oxidizing conditions found in boiling water reactors, but these are easier to remove than chromium substituted ferrites. Radionuclides in the deposits can come from non-radioactive ions that enter the coolant and are made radioactive by neutron bombardment in the core. For example, cobalt from hard facing alloys, which are used in seals and valve facings, can go from non-radioactive cobalt 59 to highly hazardous and radioactive cobalt 60 when bombarded by neutrons. Also, stable nickel 58, from high nickel alloys (e.g., Inconel), can be irradiated to produce radioactive cobalt 58.
These deposits can form on the inside surfaces (primary surfaces) of the primary loop of a pressurized water reactor, or in the steam generator, or in the piping in between. The deposits could also form on the steam generating side (secondary surfaces) of the steam generator, but there the problem is much less severe because the radioactivity is lower and the deposits are more easily dissolved. In a boiling water reactor the deposits can form on turbine blades or in any part of the cooling loop. In a high temperature gas reactor, the deposits can form on the primary cooling loop. Generally, the deposits formed in pressurized water reactors are the most difficult to remove, so if a process and composition can remove those deposits, it can also remove deposits formed in other types of reactors.
While the deposits are usually too thin to plug any of the tubing, they represent a safety hazard to personnel because of their high radioactivity. Thus, in order to inspect the cooling system and perform maintenance on it, it is necessary to decontaminate it first so that the hazard to humans is reduced or eliminated. In addition to the radiation hazard the deposits present, they also prevent the formation of a good seal when tubing must be repaired. This is done by "sleeving," inserting a new, smaller tube into the old tube and swaging the tubes together. In a steam generator it is necessary to hone a tube with an abrasive to remove the oxide layer down to clean metal in order to obtain a good seal by swaging or brazing. Because this is a time-consuming task, it increases the radiation exposure to the technician.
In spite of their thinness, (usually only about 2 to 5 microns), radioactive deposits in the cooling systems of nuclear reactors are very tenacious and difficult to remove. Many techniques have been tried to eliminate these deposits. Inhibitors have been added to the coolant system, but most inhibitors break down under the extreme conditions of temperature and radiation, and, in doing so, may form corrosive products. Continuous precipitation of the ions forming the deposits has been found to be ineffective. Many decontamination solutions which have been tried may themselves corrode the metals in the cooling system or may work too slowly to be economical. This is particularly true of concentrated reagents, which may require shutting down the power plant for several months. Speed in decontaminating is important because a generator which is shut down can cost a utility a million dollars a day in lost electricity.